Boundary lubrication, description

The Stribeck curve gives a general description for the transition of lubrication regime, but the quantitative information, such as the variations of real contact areas, the percentage of the load carried by contact, and changes in friction behavior, are not available due to lack of numerical tools for prediction. The deterministic ML model provides an opportunity to explore the entire process of transition from full-film EHL to boundary lubrication, as demonstrated by the examples presented in this section. 

An example of the velocity dependence of friction is given in Figure 5 for a boundary lubricant confined between two incommensurate surfaces.25 For the given choice of normal pressure and temperature, one finds four decades in sliding velocity for which Eq. [7] provides a reasonably accurate description. 

Tribological test results (macroscopic effects) and molecular descriptions of a boundary layer are joined together in one dependence using the a, model [16, 17]. The model is based on the application of system analysis to tribological systems. The model s main assumption is that due to the relationship between two functions f y) and (y), describing tribological processes in a given test, the coefficient o can be treated as a property of the lubricant which is not connected with only one tribotest as follows ... 

The preceding description is appropriate for the case of parallel boundary motion. However, if the motion in the lubrication layer results from the relative motion(s) of the boundaries toward (or away from) one another with velocity V, we see from (5-59) that we may simply choosenc = V/s. Finally, although the characteristic pressure could also be adapted directly from the analysis of the preceding section, its scaling is a key result of the lubrication theory, and we thus initially adopt the symbolic notation... 

The present chapter deals with frictional behavior when lubricated surfaces rub against each other. In Chapter 7 attention was called to the distinction between viscous loss in a fluid film separating two solid surfaces and the friction of the surfaces rubbing directly against one another. In this chapter we shall discuss the type of lubrication in which the rubbing surfaces that carry the lubricant participate intrinsically in the lubrication process rather than merely as the geometric boundaries of the fluid film. The treatment of lubricated friction here will be mainly descriptive detailed analysis and discussion of theoretical models for lubricated frictional rubbing is reserved for subsequent chapters. 

An important aspect of the function of compounded lubricants is to increase the load that can be carried by machinery without catastrophic damage to the rubbing components. Since the typical antiwear additives affect the viscosity of the carrier oil very little, it is not a fluid film effect that is responsible for the load-carrying augmentation. Examination of the various basic wear processes leads to the choice of the adhesive mechanism as the one most likely to respond to the action of boundary or extreme-pressure additives. The type of macroscopically observed severe wear which has this mechanistic process as its primary cause is generally designated as icu i ng (c(S. Chapter 13, Sections 13.4 and 13.6), and it is in this sense, as a description rather than a definition, that the term scuffing is used in the discussion to follow. 

---